Soft X-ray Reflection Ptychography

TL;DR

This work tackles the thickness constraint of soft X-ray imaging by introducing reflection-mode ptychography that probes the topmost sample depth without thinning. The authors implement a grazing-incidence ptychography setup using a multilayer [Si/W] Bragg reflector and zone-plate illumination at $E \approx 760\mathrm{eV}$, achieving a measured full-pitch resolution of about $45\mathrm{nm}$ (down to $30\mathrm{nm}$ under certain analysis). They validate the method with lithographically defined Siemens star and barcode patterns, and observe anisotropic resolution stemming from the tilted geometry and multilayer angular filtering. The results establish reflection ptychography as a nondestructive tool for soft X-ray materials and point to extensions toward time-resolved, coherent-imaging modes and in situ measurements with higher-coherence sources.

Abstract

Scanning transmission X-ray microscopy and ptychography have become mature tools for high-resolution, element-specific imaging of nanoscale structures. However, transmission geometries impose stringent constraints on sample thickness and preparation, thereby limiting investigations of extended or bulk specimens, especially in the soft X-ray region. Here, we demonstrate reflection geometry soft X-ray ptychography as a robust imaging mode. Instrumental feasibility and spatial resolution are established using a lithographically defined Siemens star and barcode test pattern on a multilayer substrate. We empirically demonstrate a full-pitch spatial resolution of ca. 45 nm from Fourier ring correlation analysis of the reconstructed object. The results highlight the potential of the reflection geometry for nondestructive X-ray studies of materials without the need for transmissive samples.

Figures (5)

• Figure 1: Schematic of the experimental setup. A zone plate and order sorting aperture are used to illuminate the sample with a convergent beam of coherent soft x-rays. A ptychography dataset is collected by defocusing the zone plate illumination and using a charge coupled device (CCD) to measure coherent scattering patterns from overlapping beam positions on the sample as indicated with the transparent blue ellipses. The coherent scattering pattern intensity is shown at the CCD on a logarithmic scale.
• Figure 2: Ptychographic reconstruction results obtained using CDTools, (a) reconstructed object amplitude and (b) phase distributions and the (c) amplitude and (d) phase of the retrieved probe.
• Figure 3: (a) Scanning electron microscopy image of the Siemens star with a horizontal and vertical barcode, (b) reflection ptychography image where boxed area used for 1D line-cuts are marked for the horizontal (blue) and vertical (orange) directions. The beam propagation and tilt are along the vertical and horizontal is parallel to the rotation axis. (c) Comparison between the horizontal and vertical barcode pattern as indicated in (b). (d) Comparison of the reconstruction and simulation of the star center (e) simulated transmitted signal through the $32nm$ of gold for the horizontal and vertical bar pattern, (f) line-cut plots through the barcode reconstruction, and (g) FRC curve with thresholds determining the resolution estimations.
• Figure S1: Cu $K_\alpha$ X-ray reflectivity measurement of the [Si/W] multilayer Bragg reflector used in this study.
• Figure S2: Example of one of the collected diffraction patterns used for the reconstruction. Speckles are clearly visible around the main donut pattern representing the zoneplate illumination. A brighter band exists due to the multilayer used as discussed in the main text.